In a radio system using code division multiple access (CDMA), a plurality of transmitting and receiving stations are able to communicate in the same frequency band of the radio spectrum. A spreading code is reserved for each user for the duration of a connection, enabling the user to spread the information in a base-frequency signal. A receiver of the signal, in turn, is able to identify the information sent by the user by despreading it with a despreading code corresponding to the spreading code. Advantages of CDMA include efficient utilization of the frequency band, and system security. A disadvantage is that users interfere with each other's transmissions because of lack of orthogonality between spreading codes and lack of synchronization between transmitters. Interference, in turn, affects the utilization of the capacity of a radio system and the quality of connections.
Several methods exist for interference cancellation (IC) in CDMA systems. For example U.S. Pat. No. 5,579,304 discusses serial interference cancellation (SIC). In the solution presented in the publication, in each interference-cancelling stage, the signal of one or more users is removed in a receiver from a received combination signal of all signals. One or more strongest signals are removed from the received signal in each interference-cancelling stage. This allows a user of weaker signals to be identified in later interference-cancelling stages from a signal from which users that sent the strongest signals have been removed.
Interference cancellation methods based on multi-user detection (MUD) are effective. The idea in MUD-based interference cancellation methods is to utilize the information of several other users in the detection of each user signal. In the first stage, user signals are subjected to a detection stage by multiplying a combination signal by each user's spreading code to generate first symbol estimates. The symbol estimates are generated on the basis of symbols transmitted on a pilot channel or in bursts and known to the transmitter and receiver. After the detection stage, the user signals are subjected to one or more interference-cancelling stages, during which attempts are made to improve the symbol estimates of the user signals. In each interference-cancelling stage, the user signals are respread by regenerators or respreaders by means of symbol estimates, channel estimates and user spreading codes generated in a preceding detection stage or interference-cancelling stage. The regenerated user signals are combined to generate an interference signal. Parallel interference cancellation (PIC) methods generate an interference signal and utilize the generated interference signal in various ways depending on the method. In some solutions, the interference signal generated is subtracted from the combination signal, and the residual signal generated is used in the generation of user signal estimates specified in a detection step of the interference cancellation stage. The specified user signal estimates are generated by means of correlators that receive as input, in addition to a combination signal or a residual signal, a despreading code for each user. Code generators are used to generate both the spreading code for the regenerators and the despreading code for the correlators. Specified user signal symbol estimates and a residual signal, generated from the received combination signal by subtracting the regenerated user signals from it in the interference-cancelling stage, are thus obtained as output of each interference-cancelling stage.
Prior art solutions for multi-stage interference cancellation have a drawback. The necessary interference cancellation equipment, such as regenerators, code generators and correlators, has to be duplicated for each interference-cancelling stage. This increases significantly the costs of the interference cancellation equipment, and essentially increases the complexity of the equipment.